Method of activating thermionic emitter



1959 J. E. WHITE 2,913,629

METHOD O ACTIYATING THERMIONIC EMITTER Filed May 2, 1956 PULSER 7 q i-CATHOUEBREAKDOWN EVACUATION BAKEOUT rmiigfs R.F. BOMBARDM'ENT APPLICATICN OF CATHODE PULSED'PLATE VOLTAGE HEATING INVENTORI JOHN E.WHITE BY WWW HIS ORN Y.

is required.

2,913,629 METHOD F ACTIVATING THERMIONIC TE .i John 'E.'White,'Cazenovia, N.Y., assignor to'General Electric Company, a corporation of New York Application May 2, 1956,Serial No. 582,159 6 Claims. (Cl. 315-103) This invention relates to the manufacture of electron tubes, and more particularly, to a method and apparatus for activating thermionic emitting cathodes of the oxidecoated'type.

One advantageous form of thermionic emission cathode consists of a base metal such as nickel coated with the oxide of an alkaline earth such as barium, or strontium, calcium, or a mixture of these, the coating being applied in the form of a carbonate, for example, and thermally decomposed to the oxide form. The activation of such a cathode is essentially a reduction process in which some oxygen is driven off and some free alkaline earth metal is thereby developed so that the final coating is a mixture of oxides, e.g. barium oxide, and pure metal, e.g. barium. The extent and rate of activation generally varies directly as the proportion of reducing impurity'in' the base metal alloy. Bas e metals .are generally designated active when they contain up to about .25 percent of such impurities as silicon, for example, whereas if the base metal contains as little as .01 percent of reducing impurities it is commercially designated passive, and if it contains intermediate amounts of impurities it is generally designated normal.

The other general method is the so-called electrolytic 'method. This method consists of heating the cathode and applying a high DC. voltage to theplate so as to create a strong electric field between cathode and plate which draws negative oxygen ions to the surface of thecathode, whence the oxygen escapes and leaves some free alkaline earth metal in the coating. This method also has the disadvantage that it generally takes too longto beapractical production technique where high volumevprocluction One object of the present invention therefore .is .to provide an improved method and apparatus for activating both active and passive oxide-coated cathodesjn substantially less time than'has heretofore been required.

Another object is to provide an improved method of activating thermionic emission cathodes which .will result in an improved degree of uniformity-of emissionas compared to cathodes activated according to the methods of the prior art.

Another object is to provide an improved method of activation which is sufliciently rapid to be compatible with the time schedule of high-volumeproduction tubemanufacturing equipment, so that tubes can be activated substantially as rapidly as manufactured, and in an operation which can be tied directly into the rest of the-manufacturing cycle. 7

These and other objects of the invention will be-ap- 2,913,629 7 Patented lflov. 177,1959

parent from the description which follows hereinafter,

oxygen from the surface of the cathode, applying to the plate, or other electrode spaced from the cathode, pulses of positive DC. voltage of sufficient magnitude to produce temperature-limited emission from the cathode, making the width of the voltage pulses sufiiciently small to prevent the accumulation during any one pulse of such an amount of oxygen-in the vicinity of the cathode surface as tocause destructive oxygen ion bombardment of the cathode, and making the duty factor of the pulses low enough the prevent destructive overheating of the electrodes of the tube and insure sufficient time between successive pulses to permit any gas accumulated near the surface of the cathode to diffuse away from the cathode. This treatment causesintense electrolytic action which rapidly draws oxygen ions to the surface of the cathode coating, where the oxygen is evaporated from the cathode surface by the combined effects of the accelerating field and the high temperature thermal agitation. This quickly produces the necessary reduction of a suflicient proportion of the oxide coating to give an adequate quantity of free alkaline earth metal, as required for abundant uniform thermionic emission.

In the accompanying drawing,

Figure 1 is a schematic diagram of one form'of apparatus suitable for activating a tube according to the present Invention;

Figure 2 is a flow diagram of an exemplary electron discharge tube manufacturing process, including the cathode activation method of the present invention.

Referring to the drawing there is shown an electron tube having an envelope 2 and a cathode 4 to be activated consisting of a nickel 'base coated with the oxides'of one or more alkaline earth metal, such as barium, strontium, calcium, etc., and which forsimplicity will be hereinafter referred to as barium oxide. The cathode may be heated directly or by a heater 6 supplied from a filamenttransformer 8. The tube also includes a plate 10, and may includeone or more grids 12. I i I Before activation in accordance with the invention, the tube is first subjected to the conventional processing steps of thorough outgassing, getter flashing, and sealing oif of the envelope, and is baked out, as for example'byinduction heating, commonly known as external 'bom bardment.

. following bakeout, the plate is connected through a swltch 14 to a pulser 16 suitable for providing pulses of DC. voltage of an amplitude, frequency, and duty factor as will hereinafter the described. Advantageo usly, any

that at which heat destruction would lake-place. Simuln y, P Pl Q Qf'P-C- vq tasej r m p lse are applied to the plate.

While it will be appreciated that the choice of absolute values for plate voltage, pulse magnitudaiwidth, and duty factor, as well as cathode temperature, :wiJLdependupon ode emitting area.

the tube type being activated, as will be apparent from the exemplary activation schedules set forth hereinafter, the following criteria will apply: According to the invention, the amplitude of the voltage pulses applied to the plate must besufiicient to produce temperature limited emission from the cathode. That is, the voltage gradient from the plate to the cathode must be sufficient to create an electric field adajacent the surface of the cathode such as to accelerate electrons away from the cathode surface. With tubes of the 6SN7 type, for example, successful and rapid activation was accomplished with plate voltage pulses having a magnitude in the range of 1300 to 1700 volts. Under these circumstances substantial cathode to plate currents will be drawn, in the range, for example, of from 25 to 40 amperes per square centimeter of cath- The voltage required for any given tube to establish temperature limited emission may be established by plotting I to the /3 power versus V, where 'I is cathode current and V is plate voltage, and extrapolating the resultant straight line to the point where the emission for well activated tubes of the same type becomes temperature limited at the temperature level encountered during activation.

The combined effect of the heating of the cathode to a high temperature, and the formation of a strong accelerating field at the cathode surface is to commence an intense electrolytic action in the cathode. This causes rapid diffusion of oxygen ions from within the oxide coating to the surface of the cathode. Once at the cathode surface, the oxygen is rapidly separated from the cathode by the combined evaporating effects of the elevated temperature and the accelerating field. Oxygen is thus driven to the cathode surface rapidly, yet is removed from the cathode surface substantially as fast as it is released.

The plate voltage is pulsed at a rate, according to the "invention, such as to provide intervals or rest periods the deleterious effects of overheating of the plate and/ or grids of the tube. Excellent results have been obtained in activating tubes of the 6SN7 type, for example, by using pulses having a .25 microsecond width with a repetition rate of 4000 per second.

By way of example only, and for the purpose of illustrating the principles of the invention, several sample activation schedules for various tube types are set forth below:

Type 6A U6 tubes Tubes diode-connected. Value of resistor 18 in series with control grid is 125 ohms.

Heater Plate Step Voltage Pulse Voltage Time per step=15 seconds.

Pulse wldth=0.25 microsecond.

. Pulse repetitionzate=4000 p.p.s.

Time per step=30 seconds.

Pulse width=0.25 microsecond.

Pulse repetition rate=2000 p.p.s.

Alternatively, one or more of the grids may be supplied the requisite voltage pulses instead of the plate as hereinabove described. The requisites of the applied voltage pulses are in either case the same, namely, an amplitude sufficient to create temperature limited emission from the cathode, and frequency, duty factor and pulse width such as to avoid overheating the tube electrodes and prevent accumulation of excess oxygen adjacent the cathode surface.

The advantages of the activation method of the present invention are several. First the activation procedure described has been found effective to impart abundant emissivity to the cathodes. Also, as will be apparent fromthe above schedules activation of active or normal cathodes can be accomplished very quickly, e.g. in times of the order of one-twentieth to one-thirtieth that heretofore required. Hence the activation process herein described provides a tremendous saving in time and is suitable for incorporation into a volume-production tube manufacturing cycle involving the use of high speed automatic machinery. Furthermore the method of the present invention has been found effective to activate cathodes of the passive type which could heretofore be activated, if at all, only by an extremely lengthy treatment. Thus the present invention makes feasible the use of passive cathodes heretofore impractical and thereby makes available the known advantages of such cathodes. Finally, the present activation method provides an improved degree of uniformity of emission throughout the life of the cathode, thereby enhancing the stability and reliability of tube operation.

It will be appreciated by those skilled in the art that the invention may be carried out in various Ways and may take various forms and embodiments other than those illustrative embodiments heretofore described. It is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of activating an alkaline earth oxide coated thermionic electron emitter comprising heating the emitter in vacuo to a temperature above normal operating temperature and just below the temperature of heat destruction of the emitter such as to commence intense electrolytic action in the emitter and cause rapid diffusion of the oxygen within the oxide coating and rapid evaporation of released oxygen from the surface of the emitter, intermittently applying adjacent the surface of the emitter an electric field of sulficient strength and proper polarity to cause temperature-limited emission from the emitter surface, and limiting each period of temperature-limited emission to an interval less than about 0.5 microsecond.

2. The method of rapidly activating an alkaline earth oxide coated thermionic electron emitter comprising heating the emitter in a vacuum to an above normal temperature in the range of 900 C. to 1200 C. such as to cause rapid diffusion of the oxygen within the oxide coating and rapid evaporation of released oxygen from the surface of the emitter, forming adjacent the surface of the emitter an electric field having a polarity such as to accelerate electrons away from the emitter surface, and intermittently interrupting the field at a frequency and for an interval such as to allow oxygen liberated from the emitter to diffuse away from the surface of the emitter to thereby prevent destructive oxygen ion bombardment of the emitter surface.

3. The method of activating an alkaline earth-oxide coated thermionic emission cathode in an electron discharge tube which comprises evacuating and sealing the tube, heating the cathode to a temperature above normal operating temperature and just below the temperature of its heat destruction such as to cause rapid difiusion of oxygen within the oxide and rapid evaporation of oxygen from the cathode surface, intermittently applying to an anode of the tube a voltage having a magnitude such as to produce temperature-limited emission from the cathode and draw currents therefrom in the range of 20 to 50 amperes per square centimeter of cathode emitting surface area, and limiting the duration of each application of said voltage to an interval less than about 0.5 microsecond.

4. The method of activating an alkaline earth oxide coated thermionic emission cathode in an electron discharge tube which comprises evacuating and sealing the tube, heating the cathode to a temperature just below that suflicient to cause heat destruction of the cathode so as to cause rapid diffusion of oxygen within the oxide and rapid evaporation of oxygen from the cathode surface, and applying to the plate of the tube a pulsed voltage having a magnitude such as to produce an electron extracting electric field adjacent the surface of the cathode, said pulsed voltage having a duty factor sufiiciently low to prevent over-heating of the tube parts and a pulse width sufficiently small to avoid accumulation of sufficient oxygen adjacent the cathode surface to cause destructive ion bombardment thereof.

5. The method of activating an alkaline earth oxide coated thermionic emission cathode in a discharge tube having a heater, a plate, and at least one grid which comprises evacuating and sealing the tube, connecting the grid to the plate through a grid-current limiting resistor, applying above-normal heater voltage to the heater to heat the cathode to a temperature just below that sufficient to cause heat destruction of the cathode so as to cause rapid diffusion of oxygen within the oxide and rapid evaporation of oxygen from the cathode surface, intermittently applying to the plate a voltage having a magnitude such as to produce an electron-extracting electric field adjacent the surface of the cathode and draw currents therefrom in the range of 25 to amperes per square centimeter of cathode emitting surface area, whereby to accelerate electrolytic action in the cathode and promote diffusion of oxygen ions therein to the cathode surface, limiting the frequency of application of said plate voltage and the duration of each application of said plate voltage to values sufliciently small to prevent overheating of the tube parts and avoid accumulation of sufiicient oxygen on and adjacent the cathode surface to cause destructive oxygen ion bombardment thereof.

6. The method of activating an alkaline earth oxide 7 coated cathode in a vacuum tube which comprises evacuating and sealing the tube, heating the cathode to a temperature of from 900 to 1200 centigrade such as to cause rapid diifusion of oxygen within the oxide and rapid evaporation of oxygen from the cathode surface, and applying to the tube a positive pulsed plate voltage, said pulsed plate voltage having a magnitude of from 600 to 1800 volts such as to produce temperature limited emission from the cathode, said plate voltage having a pulse width small enough to avoid destructive oxygen ion bombardment of the cathode and a duty factor sufliciently low to prevent overheating of the tube parts and facilitate diffusion of oxygen from the region of the cathode in the interval between pulses.

References Cited in the file of this patent UNITED STATES PATENTS 

